Practical color photography

Preface

IN this little work historical and theoretical data have been as far as possible omitted. My endeavor has been to supply an elementary practical manual of the various processes of Color Photography, and no method or formula has been given that has not been personally tested in practice. This has naturally limited the scope of the work to some extent, but it has not entailed the omission of any information of practical value.

E. J. WALL.

Wollaston, Mass., May, 1922

Chapter I. The Spectrum — Color Formation i

Chapter II. The Sensitive Plates — How to Prepare Them « — How to Back Them — Commercial Plates 12

Chapter HI. Color Filters — Breaking Up the Spectrum — Additive and Subtractive Filters — How to Make Filters — Commercial Filters — How to Cement Filters — Mounting and Using Filters 22

Chapter IV. The Darkroom — Safelights — How to Make Them — Desensitizing Plates for Development in Bright Light — Developers and Development — Fixing and Washing 41

Chapter V. The Camera and Exposure — Construction of a Single-exposure, Three-image Camera — Lenses — Exposures — Filter Ratios — Marking the Separation Negatives for Later Identification 50

Chapter VI. Subtractive Processes — Superimposed Carbon Prints — Theory of Carbon Printing — Sensitizing the Tissues — Printing — Development — Transfer to Final Support in Superposition — Cementing — The Raydex Process 60

Chapter VII. The Imbibition Process — Pina type — How to Make the Printing Plates — Sensitizing — Printing — Staining — Pulling the Images — Registration — Improving Imperfect Results 75

Chapter VIII. Relief Processes — The Positive — Bleaching — Staining and Printing — The Etching Process —

The Negatives — Etching, Staining and Printing — Developed Relief Processes — Working Details 84

V

VI

CONTENTS

Chapter IX. Mordanting and Toning Processes — The Diachrome Process — Manipulation — Dyes and Staining — The Copper Mordanting Process — The Vanadium Mordanting Process — The Chemical Toning Process —

The Combination of Processes 95

Chapter X. The Three-color Gum-bichromate Process — Treatment of the Paper — Preparing the Coating — Pigments — Coating the Paper — Exposure, Development and Recoating — Registration no

Chapter XI. Three-color Lantern Slides — Use of Celluloid Film — Making the Emulsion — Sensitizing — Printing — Staining — Pinatype Slides — Assembling — The Photochromoscope 117

Chapter XII. Screen Plates — History — Theory — Separate and Combined Methods — The Compensating Filter— Position of the Film — How to Make the Filters — Flashlights and Appropriate Filters — Exposure of the Screen Plates — Meters — Development — Desensitizing — Reversal of the Image — The Second Development — Intensification — Fixation and Drying — Varnishing —

Artificial Light Work — Failures and Remedies — Exhibiting Screen Plates — Projection — Printing from Screen Plates — Filters and Manipulation — Mathematics of Necessary Distances — Stereoscopic Work with Screen Plates 136

Chapter XIII. The Bleach-out Process — Fugitive Dyes — Preparation of Sensitive Material — Printing and Fixing 179

Chapter XIV. The Lippmann Process or Interference Heliochromy — History — Theory — The Nature of Light — Interference Phenomena — Preparing the Sensitive Film — Sensitizing for Color — ^The Plateholder — Exposure — Development and Fixing — Viewing the Colors .... 185

Chapter XV. The Seebeck Process, or Photography with Silver Subchlorides — Another Laboratory Process — History — Making the Paper — Printing 200

CONTENTS vii

Chapter XVI. The Diffraction Process — Diffraction Gratings — Making the Plates — Printing — Viewing — Reproduction 205

Chapter XVII. The Prismatic Dispersion Process — Construction of Apparatus — Operations 212

Chapter XVIII. Two-color Processes, Bi-packs and Tri-packs — Discussion of Definition — Methods, Adjustments and Operative Details — Kodachrome 217

Chapter XIX. Cinematography in Colors — History and

Difficulties — Proposed Methods — Future Possibilities. 230

Conversion of Weights and Measures 235

Bibliography . . . . 238

Possible Sources of Various Dyes 241

Index 243

PRACTICAL

Chapter I

Before considering the various processes of color photography, it is essential to explain the formation of color, and the difference between the additive and subtractive methods of color reproduction.

The source of all color is the spectrum or ribbon of colors formed by the dispersion of white light. When a narrow slice of white light is passed through a prism, it is spread out into a band of various colors, ranging from

Fig. I red through orange, yellow, green and blue to violet. This is nothing more than an artificially made rainbow, and while the colors enumerated above are the principal ones, they insensibly merge one into the other so that there are innumerable intermediate tints.

In Fig. I is shown a normal spectrum, giving approximately the distribution of the colors. It will be seen that there are numerous transverse lines, designated by the letters of the alphabet, which always occur in the same places, that is to say in the same color regions, no matter what the method of spectrum production may be, for there are other ways of forming spectra besides the use of a prism, such as by means of diffraction gratings. These lines are called the Fraunhofer lines, from the physicist who first mapped them out, and who assigned to them the letters by which they are known. They may be looked upon as convenient milestones or data posts, which enable us to identify any color at once. For instance, the E line occurs in the middle of the green and if we were to designate the color of a dress material or a dye as similar in color or hue to the E line, there would be no question as to its exact shade; whereas the mere term “green” conveys no definite meaning. It is very common practice to define a position in the spectrum with regard to these lines, and we may meet with such terms as B ^ C or E f F, which also define the color, as one has merely to locate these particular points to determine at once the particular color designated.

I All natural objects possess the property of absorbing I more or less of white light and reflecting the remainder, I and to this property their characteristic colors are due.

(The absorption may be general and equal for every part of the spectrum, when we obtain light of lowered luminosity or brightness which is called grey. If the absorption I is complete, then obviously we have a total suppression j of all color and light and the resultant is black. But if one region of the spectrum is absorbed more than another, that is to say, if the object selectively absorbs I part of the spectrum, the sum of what is left is color.

A concrete example of the selective and absorptive power of objects is afforded by a very simple experiment.

Nearly fill a black vulcanite dish, such as is used for developing, with a solution of ammonio-sulphate of copper, which can be made by dissolving cupric sulphate in distilled water (about a ten per cent solution will do), and adding strong ammonia until the whitish precipitate which is formed at first is redissolved. A very rich blue solution is formed, and if this is poured into the dish to the depth of about three-quarters of an inch, it will appear quite black. But if a sheet of opal glass or white cardboard is placed in the solution at an angle, so that one end rests on the edge of the dish and the other on the bottom, it will be seen that the solution is actually colored and that the color shades off from the white of the opal glass through all shades of blue to almost black at the other end. This experiment proves in the first place that there must be some reflective surface capable of sending back to our eyes the incident light, and in the second place it also shows that the thicker the stratum of colored substance the deeper the color. In the case in point, the white light incident on the liquid penetrates to some depth, and, without the opal glass, reaches the black surface of the tray. As this reflects no light, the solution appears black; but on the insertion of the glass the incident light meets its surface, which reflects the whole of the light, except that absorbed or suppressed by the liquid. It is obvious that the light actually traverses a double thickness of the solution at any point, first through one distance to the glass and then through the same thickness to the surface again, so that the depth or intensity of color is due to a double absorption.

The processes involved in the above experiment are continually taking place with all objects. Light penetrates below their surfaces to a greater or less depth, and meets a reflective layer; if the substance possesses selective absorption, then the reflected light is colored.

The absorption or suppression of a particular spectral region gives color, and this is roughly shown in the accompanying diagram. Fig. 2. It will be seen that with the exception of green, all the colors are due to the suppression of one region only, while in this case there are two absorption bands, one at each end of the spectrum.

Lsnton

Crisoeen

Purple

Violet

Blue

Blue

Greea

Cray

Black

I On the left of the above diagram is given the color . that is absorbed and on the right the residual color. It will at once be seen that while we are accustomed to talk of yellows, blues, crimsons, etc., these are not simple colors, but the sum of those spectral regions that are left when a certain band is cut out. It is also evident that if the right and left hand colors be added together the result must be white light, because we are merely restoring that which was temporarily removed. Two such colors, which, when mixed together, form white light, are said to be complementary colors. There are innurnerable possible pairs of complementary colors, for it may be said that with the exception of the greens every line or point in the spectrum has its complementary. In the case of green the real complementary lies outside the spectrum, and can only be matched by assuming that instead of the spectrum being a straight line, it is actually a circle with the red and violet overlapping, forming the magentas or crimsons, in which lie the complementaries to green.

If we take two of the complementaries shown above, number II, in which blue is suppressed, with yellow as the resultant color, and number VI, which is violet-blue, and if we assume that we can match these in pigments with lemon chrome and ultramarine, and mix these pigments together, we shall find that we obtain, not white, but a more or less dirty olive-green. This seeming paradox leads us to another fundamental fact, which is that an admixture of pigments does not give the same results as an admixture of lights. Although this is a much misunderstood subject it is really very simple, but we will defer the complete explanation till we have cleared a little more ground.

The length and breadth of the spectrum are, like a camera image, dependent entirely on the instruments used. Everybody knows that the picture made with a vest-pocket camera may include as much of a given subject as a 10x12 print; the size of the result is determined by the focal length of the lens and the size of the plate or film, and is not dependent on the subject. It is obvious that if we ignore for the time being the Fraunhofer lines and divide our spectrum from the violet to the red end into equal steps, starting with wave length 4000 for the former and limiting the red to 7000, then we have 3000 different measurable steps which actually differ one from the other in color. Although our eyes are not sufficiently sensitive to differentiate between such fine gradations, yet they are capable of recognizing a great many, about 128 in all. This being the case, the question arises as to hovTwe become conscious of color. Have we, as it were, 128 strings or wires in our eyes, each of which responds to one of these small spectrum steps, or is there some simpler system? Apparently the system that answers all theoretical requirements is a three-wire one. It has been established, principally by the researches of Young and Helmholtz, that we have but three nerve fibrils in the retina, the sensitive surface 1 of the eye, which respond to all the colors of the spect trum. By the simultaneous and equal excitation of these I three nerve fibrils we receive the sensation of white, and f those of other colors by the action of two of them.

I We may, therefore, say that there are three fundamen- I tal or primary colors in the spectrum, by the admixture I of which we can form all intermediate colors and white, f The positions of these fundamental colors are shown by the I heavy vertical lines R, G. B., in Fig. 3, and the curves show how the intermediate colors are formed. It will be seen from the diagram that the fundamental red is at about 6700, and this excites but the one sensation; but the green at 5180 not only excites the green-sensitive nerve fibril but also the red and the blue, while the blue at 4600 excites the green and red also, though but slightly. If we now take any intermediate color, such as orange at 6000, we only have to draw a vertical line through this point; the distances at which this cuts the two curves R and G give the relative amounts of red and green light required to match this particular color, and, as will be seen, this is about 70 red plus 35 green. There is another and very striking experiment, which can be performed by the aid of three projection lanterns; if these are arranged to throw three superimposed circles,

B G K

Fig. 3

the correctly colored red, green and blue-violet filters placed in them, and the three objectives fitted with iris diaphragms, it is easy to show every color of the spectrum. For instance, if we project the red disc, and cap the green and blue lenses, we have the fundamental red color sensation; now, after closing the diaphragm of the green lens to its smallest aperture, we can gradually mix green light with the red by slowly opening the green diaphragm, when the red light on the screen will be seen to turn gradually more and more orange, until, when both lenses are at full aperture, we have practically a pure yellow. Now, by gradually closing the iris of the red lens, the light will become first yellowish-green, then greenish-yellow, and finally, when the red is completely cut off, pure green of the fundamental hue. In exactly the same way, by manipulating the blue lens diaphragm, we can show the gradual transition of the pure green through all the intermediate shades of greenish-blue and blue-green to pure blue. By manipulating the green diaphragm we can show the transition from pure blue to the fundamental blue-violet, which, for the sake of brevity, is generally called blue. By mixing with this the red, we can run through the whole gamut of crimsons, magentas, pinks or purples, for all these terms are used indiscriminately to designate these colors. Finally, if all three lenses are working at their full aperture, we obtain white, assuming that all three light-sources are of equal intensity; if this is not so, the white obtained will be more or less tinged with color, which may require slight adjustment of one or another of the diaphragms.

We have here the addition of light to light, for we start out with a black screen, that is one without illumination, project red light on it, then add the green and the blue-violet, with the final product of a white screen. It will now be easy to understand how we can show a picture in colors. Let us take the very simplest example, such as pieces of glass with opaque lantern-slide binding strips pasted thereon at different angles, and we can at once grasp the formation of the final colored pattern. Assuming that we have a white circular disc illuminated by the respective fundamental colors, we place in the red Iantern a sheet of glass with a vertical stripe of opaque material. Obviously this shuts out the red light, so that we have a result such as shown in Fig. 4. At the sides all three lights are present and give us white; in the vertical stripe the red is cut out, only the green and blue-violet being present, therefore the result is a pure blue stripe on a white ground. Now let us insert in the green lantern an opaque cross with the arms at an angle w

N. C

W N. \

B

Y

Y y/c y/

/ y/w

B

N. c\

W X.

Fig. 4 of 45° to the vertical stripe; then, the green being cut out, the result is a mixture of the red and the blue-violet, which as we have seen is crimson or pink. In the center, where the image of the cross passes over the vertical stripe both the red and green are cut off and only the blue-violet shows. If we now insert a glass with a horizontal opaque stripe in the blue-violet lantern, where this lo cuts out this color we have the admixture of the red and green, that is, yellow, and in the center where all three strips cross one another all light is cut off and we have black. The final picture is a white ground with a blue vertical stripe, a yellow horizontal stripe and a crimson cross with a black center. If instead of using opaque strips we use lantern slides, which range in opacities from clear glass to complete blackness, and place one in each lantern, we shall so cut out the respective lights as to give us innumerable tints and shades of color as well as greys and blacks, and as the silver image gives the forms of the objects we have a picture in its natural colors. This is the theory of the additive process.

It is not always convenient to use a lantern or an instrument in which the three images can thus be seen, and we naturally desire to see the results on paper. We have then another proposition, but one which can be explained similarly. We start in this case with an illuminated white surface, and if we place a coating of red pigment, such as carmine, on this, the whole surface will be red. If a green pigment be now washed over the red, the color becomes a dirty indefinite shade, which might be called olive-brown; and if we use over this a blue-violet wash, the result will be black. It is obvious that our fundamental colors as used for the additive process are quite unsuitable for paper prints. But if on top of the red wash we place a yellow one, we obtain a more or less pure orange, and if we superimpose blue, not blue-violet, we shall obtain black, because the blue absorbs the orange made by the red and yellow. Blue on top of yellow gives us a more or less pure green, and in conjunction with red it gives us the crimsons. Therefore, the printing colors are red, yellow and blue.

THE SPECTRUM — COLOR FORMATION ii

The action of the ' individual colors ought to be clear from Fig. 2, but we can deal with it in another way. Let A (Fig. 5) represent the spectrum reflected from a red pigment, and B that of a yellow pigment: then if these spectra are superimposed the only colors reflected are shown in C from D | E to the red, and the sum of

Red

Orange

Blue these is orange. In the formation of greens we have a similar case: let D represent the absorption of a blue pigment, and E that of a yellow, then, superimposing these, the only light common to both is the green shown in F.

From what has been said it should be clear that we obtain our colors in the case of prints, and lantern slides as well when they are colored with pigments, by subtracting colors from white light; therefore this process is generally known as the subtractive process.

Chapter II

Every photographer knows that the ordinary plate is practically color-blind, that is to say, it reproduces the violets and blues more or less like white, and the greens, yellows and reds more or less dark or black.

As in color photography we have to reproduce reds, greens and blues as of equal value, the plates used must be sensitive to these colors; therefore, we must use the so-called panchromatic plate. Any plate can be color-sensitized by bathing for a short time in certain dye solutions. While this is not a difficult operation and the resultant plates actually have a higher color-sensitiveness than commercial plates^ which are made by the addition of the dye to the emulsion just before coating, it is not a process that can be recommended for the average worker. In the first place, the necessary dyes are very costly, and must be purchased in commercial quantities sufficient for a great many plates, although actually only a very small quantity is used; secondly, efficient drying arrangements must be installed; thirdly, control of results is extremely difficult, in consequence of the many factors involved; fourthly, bathed plates have not such a long life as commercial plates; lastly, sensitizing of plates can not be carried on successfully in a room that has been used for developing and fixing. The author’s advice to beginners in color work is to adopt commercial panchromatic plates; quite enough troubles and failures will be met with in the ordinary manipulations without adding extra worries about the sensitive media.

If it is desired to experiment in this field, the following instructions will be helpful. In the first place, it is not [ advisable to select the fastest plates, as they have, as a j rule, a greater tendency to fog than slower ones, and this tendency is always increased by bathing, particularly if | the plates are to be kept some time. It is not advisable ! to bathe less than a dozen plates at a time. A medium plate, normally clean-working, should be chosen. Before entering upon the actual bathing process, it may be as well to point out that as the plates are rendered sensitive to red it is not advisable to use red light for the darkroom illumination. It can be used, but as one can see far better with a green light of equal power it is preferable to use one of those described later on ( See P. 41 ). But the plates should not be unduly exposed even to one of these, and with a little experience it is astonishing how easy it becomes to work in total darkness, just switching on the green light as required.

While grooved troughs are the most convenient for bathing a dozen plates, they are very wasteful of solution, and so flat dishes should be used. If one sufficiently large to hold four plates at once be employed, it is almost as easy as using a trough. An interval timer, clock or watch, is an absolute essential. A grooved trough is preferable for washing. There is one extremely important detail, on which the author lays great stress, and that is that the dish used for sensitizing should have ' been used for no other photographic operation and should he kept for this sole purpose, particularly if of earthenware, as this soon acquires surface cracks into which the developing and fixing solutions enter, so that minute traces of these are extracted by the dye solutions and all sorts of curious markings and fog may be the result. Glass vessels are preferable to all others and they should be of the deep variety. In no case must metal dishes be used for sensitizing, as there seems to be some peculiar action that results in the formation of a characteristic coarsegrained fog, which increases somewhat rapidly with time.

As regards the drying apparatus, a proper drying cupboard with a constant supply of heated air is naturally the most convenient, but, failing this, the simplest arrangement is a light-tight, and as far as possible airtight, box. This should be of generous size; 24 x 12 x 12 inches is not too large for a couple of dozen } plates. The author used for a long time an ordinary metal deed box with draught-excluding rubber tape round the lid. In the center of the box should be placed a block of wood, which should be of a goodly size, for instance, for the above size box 9x6 inches, and of such a height as will permit a flat dish to be placed on top and yet allow the lid to be properly shut, with about an inch to spare above the top of the dish. This dish should be filled with desiccated calcium chloride, which can be obtained from any chemical house. It should not be in too large lumps nor in powder; about the size of small walnuts or chestnuts is right.

The plates should be arranged round the sides of the box with the sensitive surface facing the center; they must not be placed with this side towards the walls of the box or unequal drying will ensue with unequal sensitiveness and more or less fog. It is advisable to place on the bottom of the box two or three layers of clean stout blotting paper, which prevents the plates from slipping.

A far more convenient arrangement is the use of an electric dryer such as is used by hairdressers; this can be attached to any electric light socket, and if an endless box be used to confine the air currents, it will be found possible to dry a dozen plates in less than half an hour. This is a great advantage, as it may be accepted as an axiom that 'the more rapidly bathed plates are dried, the greater their keeping power and the greater their freedom from fog, with increased color-sensitiveness. The box should be about two feet long and sufficiently large to accomodate two wooden drying racks, placed one behind the other, but too much space must not be allowed around the ends of the racks. The dryer should be placed at such a distance from the end of the box that the temperature half way down the box should be from 33° to 35° C. (90° to 95° F.).

Many dyes have been suggested for panchromatizing and more formulas for sensitizing, but as simplicity and ease of attaining results are among the main features that have been kept in view throughout this work, but one formula will be given, which has proved to be entirely satisfactory in practice. The dye recommended is that known as sensitol violet, and this should be prepared in the form of a stock solution, which will keep in the dark indefinitely. The following directions, which might seem unnecessarily diffuse to the expert worker, are thus given for the benefit of those without experience. In a dry 500 ccm flask place i gram of the dye and add 100 ccm pure ethyl or methyl alcohol. Shake with a circular motion so as not to distribute particles of the dye on the upper parts of the flask; then place the flask in a water bath at about 30° C. (84° F.) and raise the temperature gradually to 65° C. (150° F.), giving the i6 flask an occasional swirl round. Then add 400 ccm cold alcohol and after shaking gently pour into the stock bottle; measure another 200 ccm of cold alcohol into the flask and swirl round so as to wash off any adherent dye solution, then add this to the stock bottle; now pour 300 ccm distilled water into the flask and after shaking well add to the stock bottle. The result will be a I : 1000 stock solution of the dye, which must he kept in the dark. It may be as well to remark here that, owing to the difficulty of making up extremely dilute solutions to exact percentages by means of English weights and measures, the experimenter in color should accustom himself to work in the metric system, and acquire the necessary weights and measures. If this be impossible, a conversion table will be found at the end of the book.

Unfortunately all the isocyanin dyes, to which class this dye belongs, are more or less sensitive to acids and are more or less decolorized by them, and when in this bleached state they have little or no color-sensitizing power. They are so sensitive to acid that even the small amount of carbonic acid present in distilled water is sufficient to decolorize them, so that it is necessary to make the water for the actual bath alkaline. Ammonia is the alkali generally used, but while it is efficient it is dangerous, unless used in very minute quantities, that is, not more than one or two drops per 100 ccm of the bath, as it induces the rapid growth of fog. Borax is a far better alkali to use, as there is less likelihood of fog with keeping. The actual sensitizing bath is:

Stock dye solution 20 ccm

Borax, pure 2 g

Dissolve the borax in the water first, then add the dye solution with constant shaking. The above quantity of solution is sufficient to sensitize about 500 square inches of plate surface, or thirty 4x5 plates.